1. Field of the Invention
This invention relates to novel thermochromic compositions, novel thermochromic paint compositions and heat-recoverable memory metal devices coated on at least a portion of the exposed surface thereof with said thermochromic compositions.
2. Discussion of the Prior Art
Couplings and the devices comprising a heat-recoverable memory metal member or driver, a non-heat-recoverable insert, or liner, and a sealant are disclosed in copending U.S. application Ser. Nos. 1,372 filed Jan. 5, 1979 and 107,656 filed Dec. 27, 1979 by. E. S. Binkley, now abandoned, the disclosures of which are incorporated herein by reference. The sealant used in these patent applications comprises a thermosetting composition containing dispersed particulate fillers. In installing a heat-recoverable device of this type, heat is applied to cause the heat-recoverable member to recover, and the sealant to soften, flow, and then cure. In heating the device, it is important that the device and substrate to which it is being applied not be overheated. One method of monitoring the temperature of the device during the installation process is to apply a thermochromic paint to at least a portion of the exposed surface of the device. The use of a thermochromic paint to indicate the temperature of the sealant or adhesive is suggested in the above-mentioned applications.
It has been discovered that for a thermochromic paint to be useful in this context, it must undergo a distinct color change at a temperature in the range of from about 165.degree. C. to about 240.degree. C. The instant invention lies in the discovery of thermochromic compositions and thermochromic paint compositions which undergo a thermochromic transition in this range. The thermochromic compositions contain basic copper carbonate and a sulfur compound as hereinafter defined. These novel thermochromic compositions or complexes of copper with sulfur-containing organic acids dispersed in a paint formulation provide a means of conveniently monitoring the temperature of a coupling during installation.
The thermochromic properties of basic copper carbonate are known, and Day in "Chemical Reviews", Volume 68, Number 6, Nov. 25, 1968 reports that this compound changes from green to black at 320.degree. C. Thus, this compound by itself in a thermochromic paint is entirely unsatisfactory for use with a heat-recoverable memory metal device as described herein.
The use of sulfur compounds with thermo-sensitive coloring materials is disclosed in U.S. Pat. No. 1,844,199. On page 4 of this patent, the use of copper nitrate with thiourea is disclosed. However, a thermochromic paint containing these two compounds has been found to have a thermochromic transition temperature of 110.degree.-140.degree. C. making such paint unsuitable for use with a memory metal device used in combination with a sealant or adhesive requiring heat to soften, flow and cure.
U.S. Pat. No. 1,924,793 discloses paints for indicating changes of temperature. These paints contain a metallic substance and a substance yielding a sulfur compound when heated, specifically thiourea or a substance yielding thiourea at elevated temperatures and selected from the group consisting of thiourea and rhodammonium. As discussed more fully below, thiourea is unsuitable for use in the novel thermochromic compositions of this invention.
U.S. Pat. No,. 3,616,898 uses temperature sensitive inks to indicate that a package to be heat-sealed has been subjected to temperatures adequate to accomplish heat-sealing of the packaging materials. The patent states that suitable temperature responsive inks may be formulated from lead oxide or lead carbonate or other compounds of multivalent metals such as cobalt, bismuth, cadmium or copper which change color in the presence of sulfur as a function of time and temperature. Such compounds can be mixed with sulfur or a sulfur-containing compound, such as a thiocarbanilide, and mixed in a vehicle or bonding agent, such as an alkyd resin varnish or ethyl cellulose, together with drying agents and anhydrous solvents. It is stated that the thermoresponsive ink is selected to have an activation temperature in the range of 320.degree. F. to 450.degree. F. (160.degree. C. to 230.degree. C.). No specific ink formulation is disclosed.
U.S. Pat. No.3,980,581 discloses thermally responsive compositions which are useful as temperature indicators because they undergo a change of state, specifically, melting followed by recrystallization, at a precise and predetermined temperature. The thermally responsive materials are solid solutions of organic compounds having analogous chemical structures, e.g. a solid solution of n-butyl sulfoxide and n-butyl sulfone. The addition of nucleating agents results in improvement in the rate of recrystallization. One of the nucleating agents disclosed is Azurite, which is further identified as "Cu(CO.sub.3).sub.2 (OH).sub.2 ".